The invention relates to a new process for the production of peracylated 1-O-glycosides of general formula I, which is cited in more detail in the claims. The process according to the invention starts from economical starting materials, provides good yields, and allows the production of peracylated saccharides with 1-O-functionalized side chains on an enlarged scale.
Peracylated saccharide derivatives are valuable intermediate products in synthetic chemistry. Pharmaceutical chemistry primarily uses such components very frequently, since many highly potent and selective pharmaceutical agents carry sugar radicals. Thus, for example, in the Journal of Drug Targeting 1995, Vol. 3, pp. 111-127, applications of the so-called xe2x80x9cglycotargetingxe2x80x9d are described. So-called xe2x80x9cmulti-antennary sugar chainsxe2x80x9d are described in Chemistry Letters 1998, p. 823. By clustering sugar units, the carbohydrate-receptor-interaction in the case of the cell-cell-interaction is considerably improved. The synthesis of galactosides with high affinity to the asialoglycoprotein receptor was published in J. Med. Chem. 1995, 38, p. 1538 (see also Int. J. Peptide Protein Res. 43, 1994, p. 477). Here, derivatized galactoses with functionalized side chains are produced, which then can be suspended on various other molecules. A good survey on the use of saccharides as a basis of glycobiology was provided in Acc. Chem. Res. 1995, 321. Also, for the synthesis of LewisX mimetic agents (Tet. Lett. Vol. 31, 5503), functionalized monosaccharides are used as precursors (see also JACS 1996, 118, 6826).
The use of derivatized monosaccharides as intermediate stages for potential pharmaceutical agents was well represented in Current Medicinal Chemistry, 1995, 1, 392. Perbenzylated-1-OH-sugar derivatives (galactose, glucose) are also used in the synthesis of heart-active glycosides (digitoxin-conjugates). The 1-O-glycosylation is carried out here via trichloroacetimidate and BF3-catalysis (J. Med. Chem. 1986, 29, p. 1945). For the production of immobilized sugar ligands (e.g., linkage to HSA), functionalized, protected monosaccharides are used (Chemical Society Reviews 1995, p. 413).
It is the purpose of a group of syntheses to introduce additional functionality into a sugar molecule via a 1-O-glycosylation reaction. Here, primarily terminal COOHxe2x80x94, amino- or OHxe2x80x94 groups are of interest, since the latter can be further reacted in subsequent steps.
The production of 1-O-glycosides is carried out in most cases according to standard methods, such as, e.g., according to the trichloroacetimidate methods described by Koenigs-Knorr, Helferich or by R. R. Schmidt [W. Koenigs and E. Knorr, Ber. dtsch. chem. Ges. 34 (1901) 957; B. Helferich and J. Goendeler, Ber. dtsch. Chem. Ges. 73, (1940) 532; B. Helferich, W. Piel and F. Eckstein, Chem. Ber. 94 (1961), 491; B. Helferich and W. M. Mxc3xcller, Chem. Ber. 1970, 103, 3350; G. Wulff, G. Rxc3x6hle and W. Krxc3xcger, Ang. Chem. Internat. Edn., 1970, 9, 455; J. M. Berry and G. G. S. Duthon, Canad. J. Chem. 1972, 50, 1424; R. R. Schmidt, Angew. Chem. [Applied Chemistry] 1986, 98, 213.]
A feature that is common to all of these methods is that the 1-hydroxyl group is converted into a reactive form that is ultimately used as a leaving group. Under Lewis acid catalysis (partially in stoichiometric amount), the actual reaction is carried out with an alcohol to form 1-O-glycoside. For such reactions, numerous examples are provided in the literature.
In the production of immunostimulant KRN-7000 (Kirin Brewery), the condensation of tetra-O-benzyl-xcex2-D-galactopyranosyl-bromide with a primary alcohol, whose hydroxyl group sits at the end of a di-hydroxy-amido-C-chain (in DMF/toluene under Lewis acid catalysis), is thus a central step (Drug of the Future 1997, 22(2), p. 185). In Japanese Patent JP 95-51764, the reaction of 1-O-acetyl-2,3,4-tri-O-benzyl-L-fucopyranose with polyoxyethylene-30-phytosterol (BPS-30, NIKKO Chem., Japan) under trimethyl-silylbromide/zinc triflate catalysis was described. In Bull. Chem. Soc. 1982, 55(4), pp. 1092-6, 1-O-glycosylations of perbenzyl-sugars under titanium tetrachloride catalysis in dichloromethane are described.
In Liebigs Ann. Org. Bioorg. Chem.; EN; 9; 1995; 1673-1680, the production of 3,4,5-trisbenzyloxy-2-benzyloxymethyl-6-(2-hexadecyloxyethoxy)-tetrahydropyran is described. Starting from 2,3,4,6-tetra-O-benzyl-D-glucopyranose, the 1-O-glycosylation is performed with use of Bu4NBr, CoBr2, Me3SiBr and a molecular sieve in methylene chloride within 60 hours.
A tetrabenzyl derivative, which contains a terminal carboxyl group that is protected as a methyl ester, is described in Carbohydr. Res.; EN; 230; 1; 1992; 117. The carboxyl group can then be released and further reacted. For glycosylation, silver carbonate is used in dichloromethane. The use of expensive silver carbonate limits the batch size and makes an economical up-scaling almost impossible. The same problem applies for the compound below, which was described in Tetrahedron Lett. 30, 44, 1989, p. 6019. Here, 2,3,4,6-tetra-O-benzyl-D-mannosyl-bromide in nitromethane is reacted with 2-benzyloxyethanol with the aid of mercury cyanide to form 1-O-glycoside. The use of mercury cyanide in pilot-plant installations is problematical in nature and can be rejected from the environmental-political standpoint.
The substance libraries for the high-capacity-screening described most recently very frequently use saccharides (Angew. Chemie 1995, 107, 2912). Here, the purpose is to have present sugar components in protected form, which carry a functional group, such as, e.g., xe2x80x94COOH, or xe2x80x94NH2, which can be reacted in, e.g., an automated synthesis. The components that are used in this respect were described by Lockhoff, Angew. Chem. 1998, 110 (24), p. 3634. Primarily the 1-O-acetic acid of perbenzyl-glucose is important here. The production is carried out over two stages, via trichloroacetimidate and reaction with hydroxyacetic acid ethyl ester, BF3 catalysis in THF and subsequent saponification with NaOH in MeOH/THF. The total yield over two stages is only 59%, however.
In the same publication, the production of a 1-O-(aminoethyl)-glycoside of the perbenzylated glucose is also described. The reaction is carried out, also starting from trichloroacetimidate, by reaction with N-formylaminoethanol under BF3-catalysis in THF and subsequent saponification in MeOH/THF. The total yield is also relatively low here; it is 45%.
A 1-O-(aminoethyl) derivative of perbenzylxylose passes through as an intermediate product in Carbohydrate Research 1997, 298, p. 173. The synthesis is very lengthy, however, since it starts from 1-bromo-peracetate of xylose. The actual 1-O-glycosylation is carried out via a 1-phenylthioether, which is reacted with 2-azidoethanol under DMTST catalysis (=dimethyl (methylthio)-sulfonium-triflate) in dichloromethane (total number of stages: 7). The total yield is not suitable for an industrial application with less than 40%.
In the survey article by R. R. Schmidt in Angew. Chem. 1986, 98, pp. 213-236, direct reactions of 1-OH-perbenzyl-glucose and -ribose with 2-haloesters and triflates are described. As a base, sodium hydride in THF or benzene is used (Chem. Ber. 1982, 115); the yields are between 40 and 55%. The use of sodium hydride in dioxane or potassium-tert-butylate in THF (both at room temperature) is also described for 1-O-alkylation with triflates (Angew. Chem. 1986, 98, p. 218). The anhydrous reaction conditions that are to be followed most strictly represent a large hurdle in up-scaling such alkylations.
All processes known to date have the great disadvantage that an up-scaling of the process cannot be achieved easily. The use of Lewis acids in 1-O-glycosylation as well as sodium hydride in 1-O-alkylation already requires anhydrous reaction conditions, which in large batches is always associated with difficulties. The working-up and disposal of reaction adjuvants (Hg/cyanide/etc.) is also a problem in many cases.
The object of the invention was therefore to provide a process with which peracylated saccharides with 1-O-functionalized side chains can be produced at a reasonable price and in an ecologically beneficial way on an enlarged scale.
The object of the invention is achieved according to the process that is indicated in the claims, with which peracylated 1-O-glycosides of general formula I 
can be produced. According to the definition of the invention, sugar1 in general formula I means a monosaccharide that is functionalized in 1-OH position, whereby in this connection, it can also be a deoxy sugar, which contains an H-atom instead of one or more OH groups. In a preferred embodiment of the invention, the sugar in general formula I means a monosaccharide with 5 or 6 C atoms, e.g., glucose, mannose, galactose, ribose, arabinose or xylose or deoxy sugars thereof, such as, for example, 6-deoxygalactose (fucose) or 6-deoxy-mannose (rhamnose).
Radical xe2x80x94COR represents the acyl group that is present in at least two places based on the monosaccharide that is used or its deoxy form, and is present accordingly in several places with use of di-, tri- or polysaccharides. As radicals R, aliphatic and aromatic groups, such as, for example, methyl, ethyl, isopropyl, t-butyl or phenyl are considered.
Radical X means xe2x80x94COOxe2x80x94 or xe2x80x94NHxe2x80x94. In the result of the process according to the invention, alcohols, carboxylic acids, or amines of general formula I are thus obtained.
Radical L can mean a straight-chain, branched, saturated, or unsaturated C1-C30-carbon chain, which optionally is interrupted by 1-10 oxygen atoms, 1-3 sulfur atoms; 1-2 phenylene groups, 1-2 phenylenoxy groups, 1-2 phenylenedioxy groups; a thiophene radical, pyrimidine radical or pyridine radical; and/or optionally is substituted with 1-3 phenyl groups, 1-3 carboxyl groups, 1-5 hydroxy groups, 1-5 Oxe2x80x94C1-C7 alkyl groups, or 1-3 amino groups; 1-3 CF 1-10 fluorine atoms. In terms of the invention, preferred radicals L are 
whereby xcex3 means the interface site to the sugar, and xcex4 is the interface site to radical X. An especially preferred linker L is the xe2x80x94CH2 group.
For the production of peracylated 1-O-glycosides of general formula I, a peracylated 1-OH sugar of general formula II 
in which sugar, R and n have the above-indicated meaning, is dissolved in an organic solvent and reacted with an alkylating reagent of general formula III
Nu-L-X-Sgxe2x80x83xe2x80x83(III),
in which Nu means a nucleofuge, L and X have the above-mentioned meaning, and Sg is a protective group, in the presence of a base and optionally a phase transfer catalyst. As a nucleofuge, for example, the radicals xe2x80x94Cl, xe2x80x94Br, xe2x80x94I, xe2x80x94OTs, xe2x80x94OMs, xe2x80x94OSO2CF3, xe2x80x94OSO2C4F9 or xe2x80x94OSO2C8F17 can be contained in the alkylating reagent of general formula III.
Protective group Sg is a common acid or amine protective group, depending on whether X means the radical xe2x80x94COOxe2x80x94 or xe2x80x94NHxe2x80x94. These protective groups are well known to one skilled in the art (Protective Groups in Organic Syntheses, Second Edition, T. W. Greene and P. G. M. Wuts, John Wiley and Sons, Inc., New York 1991).
The reaction according to the invention can be carried out at temperatures of 0-50xc2x0 C., preferably 0xc2x0 C. to room temperature. The reaction times are 10 minutes to 24 hours, preferably 20 minutes to 12 hours.
The base is added either in solid form, preferably in fine-powder or liquid form. Cesium carbonate, potassium carbonate, 1,8-diazabicyclo-[5.4.0]undec-7-ene (DBU), 1,5-diazabicyclo-[2.2.2]octane (DBN), 1,4-diaza-bicyclo[2.2.2]octane (DABCO), potassium-t-butoxide and sodium-t-butoxide, sodium carbonate, or a mixture that consists of cesium carbonate and potassium carbonate or sodium carbonate are used as preferred bases.
As organic solvents, for example, acetonitrile, dioxane, tetrahydrofuran, diethoxymethane, 1,2-dimethoxyethane, diethylene glycol dimethyl ether, benzene, formamide, hexane, toluene, dimethylformamide, dimethylacetamide, cyclohexane, CF3-benzene, diethyl ether, dichloromethane, methyl-t-butyl ether (MTB), dimethyl sulfoxide, sulfolane or mixtures thereof can be used in the alkylating process according to the invention.
As phase transfer catalysts, the quaternary ammonium or phosphonium salts that are known for this purpose or else crown ethers, such as, e.g., [15]-crown 5 or [18]-crown 6, are used in the process according to the invention. Preferably quaternary ammonium salts with four hydrocarbon groups that are the same or different on the cation, selected from methyl, ethyl, propyl, isopropyl, butyl or isobutyl, are suitable. The hydrocarbon groups on the cation must be large enough to ensure good solubility of the alkylating reagent in the organic solvent. According to the invention, N(butyl)4+-Clxe2x88x92, or N(butyl)4+-HSO4xe2x88x92, but also N(methyl)4+-Clxe2x88x92 is especially preferably used.
After the reaction is completed, the working-up of the reaction mixture can be carried out by isolation of the still protected end product and subsequent usual cleavage of the protective group to the end product of general formula I. It is preferred, however, not to isolate the still protected end product but rather to remove the solvent, to take up the residue in a new solvent that is suitable for the cleavage of the protective group and to perform the cleavage here. The procedure for cleavage of the protective group and for regeneration of the acid, amino or hydroxy group is well known to one skilled in the art.
If, for example, protective group Sg is an acid protective group that blocks the acid proton of the carboxy group, thus, e.g., methyl, ethyl, benzyl or tert-butyl, the acid is usually regenerated by alkaline hydrolysis. In the process of the invention, however, the alcoholic hydroxyl groups are also protected as esters. As a protective group for the carboxylic acid in addition to the allyl group and silyl group, the benzyl group is available. This protective group can be easily removed by catalytic hydrogenation. As a catalyst, in this case palladium (10%) on activated carbon has proven to be effective.
As hydroxy protective groups (in L), e.g., benzyl, 4-methoxybenzyl, 4-nitrobenzyl, trityl, diphenylmethyl, trimethylsilyl, dimethyl-tert-butylsilyl, or diphenyl-tert-butylsilyl groups are suitable.
The hydroxy groups can also be present, e.g., as THP-ethers, xcex1-alkoxyethylethers, MEM-ethers or as esters with aromatic or aliphatic carboxylic acids, such as, e.g., acetic acid or benzoic acid. In the case of polyols, the hydroxy groups can also be protected in the form of ketals with, e.g., acetone, acetaldehyde, cyclohexanone or benzaldehyde.
The hydroxy protective groups can be released according to the literature methods that are known to one skilled in the art, e.g., by hydrogenolysis, acid treatment of ethers and ketals, alkali treatment of esters or treatment of silyl protective groups with fluoride (see, e.g., Protective Groups in Organic Syntheses, Second Edition, T. W. Greene and P. G. M. Wuts, John Wiley and Sons, Inc., New York, 1991).
The NH2 groups can be protected and released again in a variety of ways. The N-trifluoroacetyl derivative is cleaved by potassium or sodium carbonate in water [H. Newman, J. Org. Chem., 30:287 (1965), M. A. Schwartz et al., J. Am. Chem. Soc., 95 G12 (1973)] or simply by ammonia solution [M. Imazama and F. Eckstein, J. Org. Chem., 44:2039 (1979)]. The tert-butyloxycarbonyl derivative is equally easy to cleave: stirring with trifluoroacetic acid suffices [B. F. Lundt et al., J. Org. Chem., 43:2285 (1978)]. The group of NH2-protective groups to be cleaved hydrogenolytically or reductively is very large: The N-benzyl group can be cleaved easily with hydrogen/Pdxe2x80x94C [W. H. Hartung and R. Simonoff, Org. Reactions VII, 263 (1953)], which also applies for the trityl group [L. Zervas, et al., J. Am. Chem. Soc., 78:1359 (1956)] and the benzyloxycarbonyl group [M. Bergmann and L. Zervas Ber. 65:1192 (1932)].
Of the silyl derivatives, the easily cleavable tert-butyldiphenylsilyl compounds [L. E. Overman et al., Tetrahedron Lett., 27:4391 (1986)] and the 2-(trimethylsilyl)-ethyl carbamates [L. Grehn et al., Angew. Chem. Int. Ed. Engl., 23:296 (1983)] and the 2-trimethylsilylethanesulfonamides [R. S. Garigipati and S. M. Weinreb, J. Org. Chem., 53:4134 (1988)] are used, which can be cleaved with fluoride ions. Especially easily cleavable is the 9-fluorenylmethyl-carbamate: The cleavage is carried out with amines such as piperidine, morpholine, 4-dimethylaminopyridine, but also with tetrabutylammonium fluoride [L. A. Corpino et al., J. Org. Chem., 55:1673 (1990); M. Ueki and M. Amemiya, Tetrahedron Lett., 28:6617 (1987)].
The isolation of the end product of general formula I (amine or carboxylic acid) that is obtained is also carried out according to methods that are commonly used and well known to one skilled in the art.
Thus, for example, in the case of the acid protective group, the solvent is evaporated from the hydrolysis reaction, and the residue is taken up in an aprotic solvent. By acidification with an aqueous acid solution, the pH is set at about 2-4, and then the organic phase is separated. Using crystallization or chromatography, the peracylated 1-O-glycoside can now be obtained.
The compounds of general formula I that are obtained optionally also can be converted into their salts in the usual way.
The yields of the compounds of general formula I, which can be achieved with the process according to the invention, are good. For known compounds in which a comparison with the prior art is possible, they exceed the yields of the prior art. Thus, for example, (WO 96/35700), in the reaction of acetobromoglucose with glycolic acid methyl ester under the influence of mercury oxide and mercury(II) bromide according to Koenigs-Knorr, the 1-O-methyloxycarbonylmethyl-2,3,4,6-tetra-O-acetyl-glucopyranose is obtained with a yield of 60%. The saponification to the free acid would be connected with a further loss in yield. EP 882733 also describes the production of this compound, but without yield information. According to the process of the invention, the acid is also obtained in a two-stage process. Here, the yield is 78%, however (Example 24 of this application).
The corresponding benzyloxycarbonylmethyl-2,3,4,6-O-tetraacetyl-galactopyranose is described in JP 6-271597.
In addition to the high yields, the process according to the invention also offers the advantage that it starts from economical starting materials, makes possible a scale-up of the process, and allows an easy isolation of the end products.
The starting materials are commercially available products or can be obtained easily from commercially available precursors. Tetra-2,3,4,6-O-acetyl-D-glucopyranose thus can be easily obtained from the pentaacetyl compound by partial hydrolysis with benzylamine (Organikum, 4th Edition, VEB Deutscher Verlag der Wissenschaften Berlin 1964, p. 376). In the case of Fluka, methyl-D-manno-pyranoside and methyl-D-galactopyranoside are catalog items. By acylation and cleavage of the glycoside, 2,3,4,6-tetra-O-acyl-D-mannose or -galactose can be obtained.
The peracyl-1-OH derivatives of the pentoses (ribose, arabinose), hexoses and deoxyhexoses (rhamnose, fucose) can be obtained via the sequence of methylglycoside-peracyl-methylglycoside-peracyl-1-OH-saccharide.
The compounds that are produced according to the invention are valuable intermediate products in synthetic chemistry. They can thus be used, for example, in the synthesis of carbohydrate dendrimers, for synthesis of NMR contrast media and for introducing sugar radicals into pharmaceutical agents.
The process according to the invention is to be explained in more detail below in the embodiments.
The entire disclosures of all applications, patents and publications, cited herein and of corresponding German application No. 10135098.8, filed Jul. 11, 2001, and U.S. Provisional Application Serial No. 60/305,876, filed Jul. 18, 2001, are incorporated by reference herein.
Without further elaboration, it is believed that one skilled in the art can, using the preceding description, utilize the present invention to its fullest extent. The following preferred specific embodiments are, therefore, to be construed as merely illustrative, and not limitative of the remainder of the disclosure in any way whatsoever.
In the foregoing and in the following examples, all temperatures are set forth uncorrected in degrees Celsius and, all parts and percentages are by weight, unless otherwise indicated.